I. Field of the Invention
The present invention is directed to an improved process for producing polycarbonates having improved low-temperature toughness which includes removal of the cyclic oligomers to a level less than 1%. More specifically, the cyclic oligomers are removed with a solvent having a cohesive energy density less than 90, is immiscible with water and not reactive to alkali and the cyclic oligomers recycled whereby the cyclic oligomers are hydrolyzed to monomers so that a continuous process may be employed.
II. Description of the Prior Art
According to the method in U.S. Pat. No. 3,240,755, a mixture of solvents is utilized to precipitate polycarbonate resin for the purpose of obtaining a uniform average molecular weight. This patent utilizes cohesive energy density as a tool to define the partial solvents employed in the process to obtain the desired average molecular weight polycarbonate. The patent has a recycling system but fails to recognize the presence of cyclic oligomers or to disclose the hydrolysis thereof in a continuous recycle process.
According to patent U.S. Pat. No. 3,668,181, the crude precipitated polycarbonate resin manufactured in accordance with the method of U.S. Pat. No. 3,240,755 is purified by extraction with aqueous methanol or dimethoxy-ethane or a mixture thereof.
Still further, U.S. Pat. No. 3,410,823 is directed to a method for removing contaminates from high-molecular weight polycarbonates by mixing the contaminated polycarbonates with a liquid inert non-solvent for the carbonate which is a good solvent for the contaminates in removing the contaminates with the solvent. The solvents disclosed are low boiling ketones, in particular, acetone and low boiling dialkyl ethers. Similarly, a polycarbonate purification process is disclosed in U.S. Pat. No. 3,213,060 which utilizes swelling agents such as esters of carboxylic acids or water immisicible ketones or ethers.